Previous efforts for converting biomass to liquid fuels and chemicals have focused on transforming a variety of oxygenated hydrocarbons to desirable products using condensation reaction pathways. The condensation reaction can be catalyzed using a zeolite catalyst, for example, under moderate conditions (i.e. temperatures between 80° C. and 600° C. and pressures at or slightly greater than atmospheric).
The most common process for converting oxygenated hydrocarbons to gasoline range hydrocarbons is known as the methanol to gasoline (MTG) process (Mobil Oil Corporation ca. 1980). Additionally, Mobil and ConocoPhillips have developed and patented methods for converting biomass derived carbohydrates (e.g., glucose, xylose, starch, sucrose) and sugar alcohols (sorbitol and xylitol) to similar gasoline range hydrocarbons. However, one of the major disadvantages of processing these highly oxygenated species with zeolite catalysts is the production of high yields of undesired coke, which severely harm/limit catalyst performance and final product yields.
Chen et al. developed the hydrogen to carbon effective (H:Ceff) ratio as a tool to assist in determining the suitability of oxygenated hydrocarbon feedstocks for catalytic conversion to hydrocarbons using zeolite catalysts (N. Y. Chen, J. T. F. Degnan and L. R. Koenig, Chem. Tech. 1986, 16, 506). The H:Ceff ratio is based on the amount of carbon, oxygen and hydrogen in the feed, and is calculated as follows:
      H    ⁢          :        ⁢                  ⁢          C      eff        =            H      -      20        C  where H represents the number of hydrogen atoms, O represents the number of oxygen atoms, and C represents the number of carbon atoms. Water and molecular hydrogen (diatomic hydrogen, H2) are excluded from the calculation. The H:Ceff ratio applies both to individual components and to mixtures of components, but is not valid for components which contain atoms other than carbon, hydrogen, and oxygen. For mixtures, the C, H, and O are summed over all components exclusive of water and molecular hydrogen. The term “hydrogen” refers to any hydrogen atom, while the term “molecular hydrogen” is limited to diatomic hydrogen, H2.
Zhang et al. studied the impact of the H:Ceff ratio on the conversion of various biomass-derived oxygenated hydrocarbons to coke, olefins and aromatics using a ZSM-5 catalyst (Zhang et al., Catalytic conversion of biomass-derived feedstocks into olefins and aromatics with ZSM-5: the hydrogen to carbon effective ratio, Energy Environ. Sci., 2011, 4, 2297). Zhang reported that biomass derived feedstocks having H:Ceff ratios of between 0 and 0.3 produced high levels of coke, making it non-economical to convert such feedstocks to aromatics and chemicals. By hydroprocessing the feedstock to add hydrogen, Zhang was able to produce aromatics and olefins using a ZSM-5 catalyst at yields higher than a process without hydrogenation. However, the ratio of olefins to aromatics also increased with increasing H:Ceff ratio, with the olefin yield higher than the yield of aromatics for all feedstocks. It was reported that there is also an inflection point at a H:Ceff ratio of 1.2, where the aromatic and olefin yield does not increase further. Zhang indicated that at most the yield of high value aromatic chemicals, such as benzene, toluene, and xylenes (BTX), may be limited to 24% when using zeolite catalysts according to the disclosed process.
Oxygenated hydrocarbons derived from biomass, such as carbohydrates, sugars, and sugar alcohols have a low H:Ceff ratio. A typical carbohydrate or sugar has a formula that can be represented by the formula ((CH2O)n)m where n is typically equal to 3-6 (i.e. triose, tetrose, pentose, or hexose) and m is any number between 1 (i.e. a monosaccharide) and tens of thousands for large polysaccharides. A molecule of the formula ((CH2O)n)m will have a H:Ceff ratio of 0. Sugar alcohols, likewise, have low H:Ceff ratios. For example, the C6 and C5 sugar alcohols like sorbitol and xylitol have an H:Ceff of 0.33 and 0.4, respectively, making them undesirable for condensation reactions due to the excessive amount of coke formed on the condensation catalyst.
To overcome the limitations in converting oxygen-rich (alternatively, hydrogen-deficient) biomass-derived feedstocks to hydrocarbons, biomass derived feedstocks have been converted to oxygen-deficient (alternatively, hydrogen-rich) molecules, such as monooxygenated hydrocarbons (alcohols, ketones, cyclic ethers, etc.), while keeping the carbon chain intact. The monooxygenates are subsequently converted to gasoline range hydrocarbons using a condensation catalyst. See, for example, U.S. Pat. Nos. 7,767,867, 8,017,818, 8,231,857 and U.S. patent application Ser. Nos. 12/980,892 and 13/586,499, the contents of which are incorporated herein in their entirety.
Under the described methods, the conversion to monooxygenates from the biomass-derived oxygenated hydrocarbons results in an oxygenate mixture having an overall H:Ceff ratio close to 2. The overall H:Ceff ratio is based on the combined H:Ceff ratio for all of the hydrocarbons (both oxygenated and non-oxygenated) in the oxygenate mixture. The monohydroxyl alcohols have a H:Ceff ratio of 2.0 regardless of size, while the H:Ceff ratio for cyclic ethers, ketones, aldehydes, and alkanes vary with the length of the hydrocarbon. For example, the H:Ceff for the C6 and C5 cyclic ethers, ketones, and aldehydes is 1.67 and 1.6, respectively, while the H:Ceff for the C6 and C5 alkanes is 2.33 and 2.40, respectively. The alkanes in any substantial quantity are particularly undesirable because they are largely unreactive when further processed during condensation and contribute to a higher H:Ceff ratio.
Although forming monooxygenates allows for the condensation of oxygenates without the production of an excessive amount of coke on the catalyst, the process comes at a cost. Specifically, condensation of monooxygenates leads to substantial alkane production often at yields comparable to the production of aromatic molecules. For applications where aromatic molecules are highly desirable, the significant production of alkanes reduces the total aromatics produced, thereby increasing the overall cost of the final end products. Therefore, there is a need for methods for yielding aromatics in high percentages while minimizing alkane production, methods for producing the mixture of oxygenates useful for those methods, and the catalysts used in the methods for forming the mixture of oxygenates. In addition, there is a need for the methods to also have a low coke yield.
The inventors have surprisingly found solutions for all of those needs based on refinements made to the overall oxygenate mixture. In particular, the inventors have discovered that a mixture of oxygenates having a H:Ceff ratio in the range of 0.5 to 1.7 and one or more of the following attributes provides unexpected and beneficial results to improving aromatics production: (1) more di- and polyoxygenates than monooxygenates, (2) more dioxygenates than monooxygenates, (3) more C2-4 oxygenates (especially di- and polyoxygenates) than C5-6 oxygenates (especially monooxygenates), and/or (4) little to no alkanes present.